This invention relates generally to the manufacture of kinescopes and particularly to a filter for preventing the infiltration of airborne contaminates during processing and while cooling after undergoing a frit sealing operation.
Kinescopes include a funnel having a neck permanently attached to the narrow end. A faceplate panel has a mosaic phosphor screen applied to its inside surface and is permanently attached to the wide end of the funnel. The phosphor screen includes three phosphors which luminense in different colors when struck by electrons. The side walls of the faceplate panel support a shadow mask which includes a large number of small apertures through which the electrons pass prior to striking the phosphor screen to obtain proper color-selection of the visual display. Accordingly, the faceplate must be fully assembled and mated with the shadow mask before being permanently attached to the kinescope funnel.
In hermetically attaching the faceplate and the funnel, a devitrifiable frit material is applied to the edge of either the faceplate or the funnel and the two pieces are then placed physically together. The frit material is then devitrified by a frit cycle in which the assembly is subjected to a high temperature, typically in the order of 400.degree. C. to 500.degree. C. After the completion of the frit cycle, the assembled kinescope is removed from the oven, or lear, and cooled. The other kinescope components, such as the electron gun, are inserted into the neck after cooling and, therefore, the neck must be left open. However, because the kinescope is fritted at a high temperature, the air within the envelope also raises to a high temperature. Accordingly, during cooling ambient air is pulled into the envelope and airborne contaminants can enter the envelope. These contaminants may lodge on the shadow mask and block the apertures, adversely affecting the visual display produced when the tube is in operation. The shadow mask is enclosed in the envelope and, therefore, is inaccessible and can not be inspected or tested for the blocked aperture condition prior to completely assembling the electron gun and other components to the completed kinescope. It, therefore, is necessary to prevent the infiltration of contaminants into the envelope.
In the prior art, this problem had been addressed by efforts to filter the air so that airborne contaminants which can cause the blocked apertures do not enter the envelope. To date, such efforts have not been particularly successful because the filter must be capable of withstanding the fritting temperature making selection of the filter material very difficult. Another difficulty with the prior art arises because the coefficient of expansion of the filter typically is substantially different from that of the envelope neck. Accordingly, a filter which properly seats and seals in the neck at the fritting temperature frequently allows air to leak around the filter during cooling toward room temperature because the difference in coefficient of expansion results in different rates of contraction. Efforts to overcome this problem by oversizing the filter to fit snugly at all temperatures result in a problem because the filters are difficult to remove after the assembly has cooled.
The instant invention addresses these problems by the provision of a filter which is temperature insensitive in that air tight seating and a positive holding of the filter in the kinescope neck exist throughout a wide temperature range.